An anti-reflection film generally employed is a multi-layered film in which a plural number of transparent layers made of metal oxide are superposed one on another. The multi-layered film is usable for all the wavelength region of visible light. On the other hand, a monolayer anti-reflection film cannot effectively lower the reflection of light in a wide wavelength region such as visible light, while it is effective for lowering the reflection of monochromatic light.
In the multi-layered film, increase of the number of the layers enlarges a wavelength region of light for which the film can be applied. Therefore, the known anti-reflection film has three or more layers formed by physical or chemical deposition. However, the multi-layered anti-reflection film is produced by a complicated process comprising a number of deposition procedures corresponding to the number of the layers. In each deposition procedure, the conditions are precisely controlled so that each layer may have predetermined refractive index and thickness. Since the production process of the multi-layered film is very complicated as described above, it costs a lot and has poor mass-productivity. Further, it is very difficult to produce a large multi-layered film in the known manner.
An anti-reflection film in which its refractive index gradually changes at the interface between a substrate and air is also known. In Japanese Patent Provisional Publication No. 2(1990)-245702, SiO.sub.2 having a middle refractive index between those of the glass substrate and MgF.sub.2 is used to form an anti-reflection film. Fine particles of SiO.sub.2 and MgF.sub.2 are mixed and applied on the glass surface to form a coating film in which the mixing ratio changes gradually in the direction of the thickness so that the refractive index may gradually change. According to the descriptions of Japanese Patent Provisional Publication No. 2(1990)-245702, the film having the above structure effectively lowers the reflection.
Japanese Patent Provisional Publication No. 5(1993)-13021 discloses an anti-reflection film consisting of two layers containing fine particles of SiO.sub.2 and MgF.sub.2 dispersed in ethyl silicate. However, in the disclosed film, the first layer has the mixing ratio (MgF.sub.2 /SiO.sub.2) of 7/3 and the refractive index of 1.42, while those of the second layer are 1/1 and 1.44, respectively. This refractive index difference is not enough to effectively lower the reflection.
Japanese Patent Provisional Publication No. 7(1995)-92305 discloses an anti-reflection film comprising fine particles having the refractive index of 1.428. The upper part of the film comprises the fine particles and air to have a low refractive index, and many fine concaves and convexes are provided on the surface. On the other hand, the lower part of the film comprises only the fine particles. The fine particle consists of core part and shell part, and the core part is made of, for example, methyl methacrylate, methacrylic acid, trifluoroethyl methacrylate or N-iso-butoxymethylacrylamide. The shell part is made of, for example, styrene, acrylic acid, or butyl acrylate.
Japanese Patent Provisional Publication No. 7(1995)-168006 discloses another anti-reflection film. The upper part of the film comprises fine particles (e.g., MgF.sub.2) and air to have a low refractive index, and many fine concaves and convexes are provided on the surface. The intermediate part of the film comprises only the fine particles to have a middle refractive index, and the lower part comprises the fine particles and a binder.